It is proposed to continue the analysis of genetic interactions in a bacteriophage system consisting of a satellite virus (equals P4) and its helper (equals Phage P2 or P2-relatives). The main aim is to elucidate the genetic control mechanisms that couple the expression of satellite and helper genomes. Proposed studies concern: (1) the mechanism(s) of activation of P2 genes by P4 (alloactivation and derepression); (2) the ability of P4 to ignore termination signals for gene expression (polarity suppression); (3) the induction of P4 prophage by P2; (4) gro, a bacterial gene essential for late P2 gene expression, and its role in P4 - P2 interactions; (5) the nature of the "incompetence" of helper genomes deficient in their interactions with P4; (6) a search for P2 - P4 tandem genomes; (7) a search for P2 - P4 recombinants; (8) contributions to the genetic mapping of P4; (9) the fertility reduction for F or R factor carrying bacteria, caused by P4 prophage.